The invention relates to a device and a method for the presentation of static or moving images by means of a laser light source and a projection screen. Such methods make it possible to present photographs, video data or also numerical values on large areas. Due to the almost unlimited depth of focus of the laser light source, the application is not limited to plane projection screens.
The projection screens used in projection methods usually have diffusely dispersive, i.e. non-specular surfaces. When such a surface is irradiated with the light of a laser, there occurs a formation of speckle interferences. Speckle interferences are always formed when two coherent wave trains of the laser light are reflected on closely neighbouring points. If that reflecting structure is smaller than the resolving power of the eye, such wave trains are imaged on the retina of the viewer in one point where they form an interference due to the high coherence length of the laser light. These interferences are the reason why there occur brightness differences in a uniformly illuminated beam spot that are perceived as granularity or a speckled pattern. When the viewer moves so far away from the beam spot that the pattern cannot be resolved anymore, the beam spot appears with a brightness distribution that is varying in time or pulsating. Thus, speckle interferences do always occur when the reflecting or transmitting surfaces have a structure size within the range of the light wavelength.
For the laser projection of static or moving images, the high brilliance of the laser light is of importance in the first place. In this connection, brilliance means the number of photons per phase-space cell, i.e. per wavelength range, per space coordinate and per solid angle element. The large coherence length of the laser light that causes the undesirable speckle effect is of subordinate importance to projection methods.
For minimizing the speckle effect, it is known from the prior art to either dissolve the coherence of the laser light or to reduce the speckle contrast by means of a sufficiently quick time-variation of the speckle interferences within the integration time of the eye.
It is a simple method for destroying the coherence to lead the laser light through a rotating diffuser. For example a glass pane with a rough surface is suitable as diffuser. When the diffuser is located at the focus of the laser beam, statistic phase variations are inserted into the beam while the spatial coherence is maintained. So the beam can continue to be focussed on a point. When the unfocussed beam is led through the diffuser, both the spatial and the time-related coherence are dissolved.
DE 101 18 662 A1 discloses a projection screen in which, by means of volume dispersion of the laser light in a layer with a constant thickness applied on the projection screen, the coherence of the reflected laser light is dissolved so that the speckle effect is avoided. According to the instruction incorporated in DE 101 18 662 A1, for example polytetrafluor ethylene is suitable as diffusing layer. The thickness of the layer is adapted to the coherence length of the laser light in such a manner that the speckle effect is reduced by a desired degree. Advantageously, the thickness of the layer is chosen larger than one tenth of the coherence length. Therefore, that projection screen produces satisfactory results only with laser light sources with a low coherence length. It is another disadvantage that changes in the projector unit always require also changes in the projection screen.
It is known from U.S. Pat. No. 5,272,473 to couple a projection screen to an acoustic source so that the acoustic waves produced by the acoustic source excite vibrations of the projection screen. The wave trains reflected by the vibrating projection screen produce different speckle interferences at every point of time. When the vibration frequency is chosen large enough, these different speckle interferences are averaged during the integration time of the eye. That way, the contrasts between interference maxima and interference minima are balanced. Thus, the speckle contrast is reduced. In this connection, the speckle contrast is defined as the mean square deviation of the intensity of every place in the illuminated item from the mean value, nominated to the squared mean value. However, it is a disadvantage of this method that there are forming standing waves in the projection screen and speckle interferences occur with an unchanged intensity on the wave nodes of the projection screen.
It is known from JP 2 000 81 602 A to use a projection screen having a structure similar to that of a liquid crystal display. The irradiated laser light is reflected on the molecules of the liquid crystal in the same manner as on a conventional projection screen. If, however, a high-frequency low-voltage signal is applied to the liquid crystal, the liquid crystal molecules vibrate at the frequency of the signal applied. That way, there are produced speckle interferences varying in the same way that again are averaged during the integration time of the eye. Disadvantages of this method for avoiding speckle interferences, however, are the technological limitations with regard to the dimensions and the fact that such projection screens cannot be bended or rolled.
It is the underlying technical problem of the invention to set forth a device for the presentation of static or moving images by means of a laser light source and a projection screen in which there do not occur any speckle interferences or occurring speckle interferences are reduced to such an extent that they are not perceived as irritating anymore. The projection screen shall not be subject to any limitations as to form and size and shall be universally usable with all kinds of laser light sources. Moreover, speckle interferences shall be uniformly suppressed on the entire area of the projection screen.
Beyond this, the projection screen shall also be easily combinable with a contrast-increasing coating.